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2Dalton’s Atomic TheoryJohn Dalton’s theory based on experiments in early 1800’sAll matter is made of tiny particles “atoms”Atoms cannot be created, divided, destroyed or changed into other types of atomsAtoms of the same element have identical propertiesAtoms of different elements have different propertiesAtoms of different elements combine in whole-number ratios to form compoundsChemical changes join, separate or rearrange atoms in compounds

3Cathode Ray TubesA cathode ray is a ray of light traveling in a vacuum (no other particles inside)The ray travels from one metal plate to another as the plates are connected to electricityCathode rayMetal plate (cathode) releases streamMetal plate (anode) to which stream travels

4Cathode Ray Tubes & ChargeIn the late 1800’s, JJ Thomson put charged plates outside the tubeNegatively charged plate-+Ray is deflected away from negative plate and towards positive platePositively charged plateIt made no difference what type of metal he used in the tube—all material produced this stream that curved towards the positive charge

5Thomson’s conclusionsThe evidence from Thomson’s work showed that there was something negatively charged in atomsSince all types of metal produced the same result, the negative charge is in all types of atomsSince atoms were overall neutral, if there was a negative charge there had to also be a positive chargeIn 1897, Thomson announced that the rays were electrons and they had a negative charge

6Theories changeThomson’s evidence showed Dalton’s idea of solid, uniform atoms was incorrect.Eugene Goldstein conducted experiments to label the positive part “protons” and determined it has the same charge as the electron (with opposite sign) but is 1837 times heavier!Thomson developed the “plum pudding” model.Since most of us aren’t familiar with plum pudding, you can think of it as a chocolate cookie dough theory

7Thomson’s Theory The “chips” are the negative electrons.The “dough” is the positive portionThe “chips” are stationary and don’t move within the “dough”Remember, officially this theory is called “plum pudding” but it’s the same idea!

8Gold Foil ExperimentHans Geiger performed experiments in the early 1900’s where he bombarded very thin gold foil with radioactive particles (alpha particles “”)They expected these relatively heavy particles to go through the atoms with a small deflection

10What did he see?Most of the alpha particles passed straight through with no deflectionThese particles did not run into anythingSome did deflect slightlyThese particles ran into something much smaller than themselvesA few were reflected back the direction they came fromThese particles ran into something very dense

11What did that mean? Atoms are mostly empty spaceElectrons (the smaller particles) were the cause of the small deflectionsThere must be a small area of the atom with most of its mass (the protons) that caused the reflections.He called this small, dense area the nucleus

12A third particleThe protons and electrons could explain the charges of the various parts of the atomThey could not explain the total mass of the atomsNeutrons were proposed in 1920’s but not confirmed until 1932 by James ChadwickNeutrons had mass similar to protons and no charge. They were located in the nucleus

13More changes to the theoryNiels Bohr performed experiments with hydrogen atoms & lightHe determined that electrons are in levels according to how much energy they have and that only certain energy amounts were allowed.

14The Bohr ModelIt consists of the nucleus with protons & neutrons and electrons in concentric orbits (circles) outside the nucleusThe circle closest to the nucleus contains the lowest energy electronsThe first level can hold 2 electron, then the next two levels can each hold 8 and then levels farther out can hold 18.

15Pictures of the Bohr ModelsElectronProtonNeutronHydrogen-1Helium-4Lithium-6

16Use of the Bohr Model nowWe no longer believe electrons are in concentric circles, but this is still a convenient way to show energy levels on 2-dimensional paper

17Modern Atomic TheoryIn the 1920’s, Bohr’s research lead the way for the study of quantum mechanics (the study of tiny particles)Modern atomic theory uses calculus equations to show how the subatomic particles act as both particles and wavesThese equations show the most probable location of electrons in the atom (known as atomic orbitals)